The idea here is to define the ampere as N electrons per second. This may make that possible. The number is around 6.241 ?-- 10^18 electrons per second. Direct counts of electrons allow a precise, repeatable way to define an amp.
The goal is to define the fundamental units from measurable properties of the universe, so that reproduceable standards can be constructed. That's been achieved for time and length, but not mass. You can buy an atomic clock that gets its time measurement from the definition of the second. (HP used to make those, but that business was sold off from Agilent in 2006.) There's a method with a Kr-86 light source and interferometers to count out a meter in wavelengths of light. But there's no corresponding standard for mass. Mass is tied to a physical 1Kg weight stored in France, and everything has to be traced back to that, with each successive derived standard kilogram a little less accurate.
A kilogram ought to be defined as N atoms of something, but atom counting isn't quite good enough yet. There's a plan to define mass through the Planck constant, which means tying the standard of mass to the standard of current.
Three fundamental units are sufficient to lock down all the other units, and this is a step towards doing that.
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